Take a look at this instructional video and learn how to fit a projected image onto a target surface with Johnny Chung from Carnegie Mellon University. Typically, you'd need to use a screen to be directly in front of the projector and at a very specific orientation to get an undistorted image. What this tutorial is trying to accomplish is to place a screen at any location that is convenient and then calibrate the projector onto the target surface. This technique is used to automatically discover the locations of surfaces in the projection area using embedded light sensors.